This invention relates to internal combustion ignition systems and more particularly to plasma ignition devices.
As is well known in the art, the plasma ignition concept improves the ability of an internal combustion engine to operate on lean fuel-air mixture with ensuing gains in fuel economy and reduced vehicle cost by way of emission equipment reductions. A low energy plasma igniter for internal combustion engines as disclosed in U.S. Pat. No. 4,471,732 to Tozzi, uses an electrical pulse forming network to generate a low energy plasma useful in igniting a lean fuel air mixture.
As is shown in U.S. Pat. No. 4,398,526 to Hamai et al., a prior art plasma ignition system may require an additional voltage applied to an ignition plug to facilitate the flow of plasma energy. In Hamai, a high frequency oscillating voltage is supplied to the spark plug prior to the plasma current flow thereby inducing multiple sparks at the ignition electrodes of Hamai et al. Other patents and publications disclosing plasma ignition systems include U.S. Pat. No. 4,739,185 to Lee et al., U.S. Pat. No. 4,672,928 to Hartig, U.S. Pat. No. 4,448,181 to Ishikawa et al., and U.S. Pat. No. 4,336,801 to Endo et al., U.S. Pat. No. 4,317,068 to Ward et al., U.S. Pat. No. 3,842,818 to Cowell et al., U.S. Pat. No. 3,842,819 to Waterson et al., "Pulsed Plasma Ignitor for Internal Combustion Engines", Fitzgerald, Society of Automotive Engineers, No. 760764, "An Investigation of a Coaxial Spark Igniter with Emphasis on its Practical Use", Clements et al, Combustion and Flame, Vol. 25, p. 189, "Design of a Plasma Jet Ignition System for Automotive Application", Asik et al., Society of Automotive Engineers, No. 770355. None of the systems disclosed in the above references includes any means to decouple the spark event from the plasma flow event.
Conceptually, in order for plasma flow to take place, a high voltage arc is supplied to the ignition electrodes. Once the voltage across the electrodes exceeds the breakdown voltage, the actual voltage across the electrodes will drop from approximately 20,000 volts to 500 to 3,000 volts when the arc is established across the electrode gap. To generate the plasma flow, the ionized molecules in the immediate vicinity of the arc are stimulated to an excited state or ionized state, thereby providing charge carriers and thus a lower resistance path for the flow of current. After creation of the ionized path between the spark plug electrodes, a current pulse is supplied to the electrodes and plasma flow will occur across the gap.
One of the difficulties experienced with plasma flow driver electronics is the requirement that most of the electronic circuit components must be capable of withstanding fairly high voltages on the order of 1000 to 3000 volts. Additionally, transformer windings and capacitor sizes are directly impacted by the voltage and current requirements of such a system, commonly referred to as transformer volt-seconds capability. A device which reduces the sustaining voltage necessary for plasma current to flow between electrodes of an ignitor gap would result in lower cost with regard to the voltage tolerances of circuit components, higher efficiency in regard to power requirements for inducing plasma flow, and reduced radiated electromagnetic interference. Reductions in maximum voltages requisite to induce plasma flow will also directly affect the volume or size of transformers required in the driver circuitry. The volume of components necessary to deliver the voltage and current to the electrodes or ignitor gap is directly related to transformer volt-seconds capability.